There are several known methods for protecting computer data. One such method is to perform periodic batch back-ups of either an entire hard disk drive or selected files on a hard disk drive. Typically files may be selected based upon a file directory tree or other criteria, such as hard-coded filenames or filenames with wildcard characters. The data is typically written to a large capacity storage device, such as a tape-drive, connected directly to the computer system. Some batch back-up systems, however, such as the system described in U.S. Pat. No. 5,133,065, permit data on computers connected to a computer network to be backed-up onto a centralized back-up device on the network. Where batch back-ups are used, it is usually recommended that disk-wide back-ups of data be performed infrequently, such as monthly, and that back-ups of new or modified files be performed frequently, such as daily.
A disadvantage of batch back-up systems is that the stored data is often out of date. Even nightly back-ups do not protect data accumulated since the last back-up. In certain businesses, such as banking and financial industries, the loss of even an hour of transactions can have serious repercussions. Another disadvantage of batch back-up systems is that typically the entire selected file is backed-up even if only a portion of the file has been modified. If the batch back-up system is operating over a network, valuable network resources are wasted transferring unchanged data.
Another known method for protecting data is to duplicate (or mirror) all data write operations occurring on a primary device onto one or more secondary (back-up) devices. In systems utilizing this method, the data storage control unit for the primary device (such as a disk controller) is directly connected to either the secondary device itself or the control unit for the secondary device.
This type of data protection has been implemented using Redundant Array Inexpensive Direct access storage device (RAID) drives. A RAID drive is in essence a package of multiple, inexpensive disk drives. Mirroring has been accomplished by configuring the RAID drive controller to write the same data to two separate disks in the RAID drive.
Mirroring techniques are also used on fault tolerant computer systems. Fault tolerant computer systems have been available for mini-computers and mainframes for years, offering survival of any single point of failure in the system. These systems, however, often require expensive, redundant hardware, additional hardware for connectivity and frequently require specialized (often proprietary) operating systems.
one disadvantage of all known real-time mirroring systems is that none provides a granularity of mirroring smaller than a disk, partition or volume set. Also, none of the known real-time mirroring systems provides for mirroring across a local or wide area network.